1. Field of the Invention
The present invention relates generally to methods and apparatuses for imparting fiber twist, and more particularly, to methods and apparatuses for imparting mechanical twist in optical fiber to reduce polarization mode dispersion (PMD) and to ensure optical fiber quality.
2. Technical Background
Light traveling in an optical fiber has two polarization modes. For optical fibers that are perfectly circularly symmetric in both geometry and internal and applied stress, operation at a wavelength or in a wavelength range which is regarded as “single-moded” actually supports two orthogonal polarization modes, wherein the two polarization modes are degenerate, propagate with the same group velocity and have no time delay after traveling the same distance in the fiber. However, in practice, optical fibers are not perfectly circularly symmetric. For example, imperfections such as geometric and form deformation and stress asymmetry break the degeneracy of the two modes. As a result, the two polarization modes propagate with different propagation constants. The difference between the propagation constants is called birefringence. Polarization mode dispersion (PMD) occurs in an optical fiber as a result of small birefringence induced by deviations of the fiber's core from a perfectly cylindrical shape, asymmetric stresses or strains, and random external forces acting upon the fiber. PMD is well known to severely impair transmission of optical signals at relatively high bit rates.
Various attempts to reduce PMD have been made and involve imparting a spin rotation into the fiber during manufacturing. One prior art method of reducing PMD involves spinning the preform during the fiber drawing process. Another method of combating PMD is to deliberately spin the fiber as it is drawn from the preform, so that a mechanical spin becomes “frozen” into the fiber as it cools. The resulting rotation of the birefringence axis in the fiber produces continual mode-coupling between the orthogonal polarization modes of a carried signal, thereby inhibiting the accumulation of a significant phase lag between the two modes, and consequently causing a significant reduction in the fiber's PMD.
A method as specified in the opening paragraph is known from United States Patent U.S. Pat. No. 6,324,872, wherein the drawn fiber is caused to pass over a roller whose rotational axis can be canted, so that the pulley can be caused to rock back and forth about an axis perpendicular to its rotational axis. The rocking motion of the roller produces a spin in the fiber along a substantial portion of its length. In particular, portions of warm fiber which are spun in this manner will become endowed with a permanent spin as their constituent material subsequently cools. This “spin” travels up the fiber to where the fiber is essentially in a malleable state, thereby solidifying the spin in the fiber as the fiber cools.
In addition to “spin”, another metric regarding optical fibers that is measured during the manufacture is “twist”. The spin and twist metrics are related, and some may use such terms interchangeably. However, as used herein, spin refers to the rotation introduced into the molten optical fiber, whereas twist refers to the rotational twist which is imparted to the cooled optical fiber. As discussed above, spin imparted into the molten fiber is permanently fixed when the fiber is cooled. Twist refers to the mechanical rotational force imposed on the optical fiber after it has cooled. Twist introduces torsional stresses on the fiber, and extreme amounts can cause microscopic cracks, and ultimately physical destruction of the fiber. Twist can be removed from the fiber by untwisting the fiber or allowing the fiber to relax, e.g. by hanging a very long length of fiber and allowing the fiber twist to “untwist” itself.